1. Field of the Invention
The present invention relates to a method and apparatus for treating an exhaust gas with organic compounds and more particularly, a method and apparatus for treating an exhaust gas containing volatile organic compounds (VOCs).
2. Description of the Prior Art
In semiconductor or liquid crystal display fabrication processes, exhaust gas discharged often contains large amounts of volatile organic compounds, such as isopropyl alcohol (IPA), acetone, methyl ethyl ketone (MEK) and the compounds with high boiling points (for example, dimethylacetamide, DMSO, NMP etc).
Presently, commercial technology for treating exhaust gas containing VOCs from a semiconductor fabrication process are mainly: rotary concentration followed by incineration, activated carbon adsorption and wet scrubber. A large amount of energy is consumed by the rotary concentration followed by incineration, while frequent replacement of activated carbon is required for the activated carbon adsorption, thereby increasing operation costs for both processes. Furthermore, it is observed that the removal efficiency of VOC in an exhaust gas gradually is reduced by the high boiling point compounds (such as the major component of the stripper when the rotary concentration followed by incineration is used. This is because the adsorption ability of the adsorption material used in rotary concentration demolishes, which brings about further decreases of the process efficiency for treating pollutants.
High boiling point substances used in the semiconductor industry are all water soluble, and hence they can be removed by wet scrubber. The wet scrubber transfers the VOC in an exhaust gas to liquid phase by the scrubbing liquid. However, large amounts of clean water are required to maintain high process efficiency, creating even more problems when water shortage and water pollution are not yet resolved in the semiconductor industry.
Conventionally, oxidizing agents are directly added in the wet scrubber for removing organic compounds in the scrubbing liquid, wherein the oxidizing agents will easily escape into the exhaust gas through the scrubber and the utilization of the agent is also low. These result in unwanted secondary pollution. When the reaction rate of the oxidizing agent with the pollutants is slow, process efficiency is decreased.
Ozone exhibits excellent oxidation capibility among the commercially avialable oxidizing agents. However, it is commonly used in water supply system and chemical processes, due to high production costs. The majority of ozone application in the field of air pollution treatment mainly focus on the labortary research. The actual engineering application for air pollution is very limited. Prior art related to the use of ozone in treating water and wastewater is found in EP 0859746, EP 0791101, EP0495707, U.S. Pat. Nos. 53,645,377, 5,851,407, and 5,637,231 etc. The use of ozone to treat water and wastewater mainly involves disinfection, or removal of some organic substances as a pretreatment for the follow-up process. Hence, high removal efficiency for organic substances is not required. Conventional ozone reactors are designed as a single reactor combined with an aeration tank, without controlling the water quality, such as pH and bicarbonate concentration in the water. The removal efficiency for organic substances and the utilization rate of ozone are low, resulting high concentration of residue ozone in exhaust gas. Hence, it is necessary to install an ozone destruction unit at the exhaust gas outlet. In order to increase the efficiency of transporting gas ozone to liquid phase, U.S. Pat. Nos. 5,851,407, 5,637,231, and 5,989,439 disclose a method using high pressure or venturi injector to quickly dissolve gas ozone. However, this causes wastage of ozone when reaction rate of the organic substances with ozone is low. Also, maintenance of the equipment is not simple.
In order to overcome the above problems, an object of the invention is to provide a method and apparatus for removing VOCs from exhaust gas.
It is another object of the invention to provide a method and apparatus for treating VOCs exhaust gas with lower operational costs.
Another object of the invention is to provide a method and apparatus for treating VOCs exhaust gas that is not affected by high boiling point substances.
Another object of the invention is to provide a method and apparatus for treating exhaust gas that greatly reduces both the amounts of required clean water and wastewater production.
Another object of the invention is to provide a method and apparatus for treating exhaust gas that exhibits high efficiency of VOC removal and ozone gas use.
In order to achieve the above objects, the invention provides an apparatus for treating an exhaust gas containing volatile organic compounds, comprising: a wet scrubber for receiving the exhaust gas thus transporting the organic pollutants in the exhaust gas into a scrubbing water; oxidation unit for receiving the scrubbing water from the wet scrubber, thereby introducing an oxidizing agent containing ozone into the scrubbing water so that oxidation occurs; and a recyling device for recyling the scrubbing water from the oxidation unit to the wet scrubber, where the pollutants in the exhaust gas are absorbed in the scrubbing water.
The invention also provides a method for treating an exhaust gas containing volatile organic compounds, comprising: introducing an exhaust gas into a wet scrubber, so that the organic pollutants in the exhaust gas are absorbed by the scrubbing water; pumping the scrubbing water containing the pollutants into at least one oxidation tank, thereby causing the oxidation of the pollutants and an oxidizing agent containing ozone; and introducing the scrubbing water after the oxidation reaction into the wet scrubber, where steps (a) and (b) are repeated.
A primary feature of the invention is the combination of ozone oxidation and conventional wet scrubbing. In the process for treating exhaust gas provided in this invention, by using highly efficient multiple oxidation reactors and controlled operational conditions, utilization rate of oxidizing agent and removal efficiency for organic pollutants are both improved. Problems such as secondary pollution caused by oxidizing agent and the hindered reaction rate between oxidizing agent and pollutants caused by the increase of bicarbonates are avoided. The method and apparatus provided in the invention are particularly useful in treating exhaust gas from semiconductor and LCD processes which contain VOCs that is highly soluble in water.